Improved Strength and Toughness of Carbon Woven Fabric Composites with Functionalized ...

18 Pages·2014·3 MB·English

Materials 2014, 7, 4640-4657; doi:10.3390/ma7064640
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materials
ISSN 1996-1944
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Article
Improved Strength and Toughness of Carbon Woven Fabric
Composites with Functionalized MWCNTs
Eslam Soliman 1, Usama Kandil 2 and Mahmoud Reda Taha 3,*
1 Department of Civil Engineering, Assiut University, Assiut 71515, Egypt;
E-Mail: [email protected]
2 Polymer Nanocomposites Center, Egyptian Petroleum Research Institute, 1 Ahmed El-Zomor
Street, Nasr City, Cairo 11727, Egypt; E-Mail: [email protected]
3 Department of Civil Engineering, University of New Mexico, MSC01 1070, Albuquerque,
NM 87131, USA
* Author to whom correspondence should be addressed; E-Mail: [email protected];
Tel.: [phone]; Fax: [phone].
Received: 28 February 2014; in revised form: 11 April 2014 / Accepted: 4 June 2014 /
Published: 18 June 2014

Abstract: This investigation examines the role of carboxyl functionalized multi-walled
carbon nanotubes (COOH-MWCNTs) in the on- and off-axis flexure and the shear
responses of thin carbon woven fabric composite plates. The chemically functionalized
COOH-MWCNTs were used to fabricate epoxy nanocomposites and, subsequently, carbon
woven fabric plates to be tested on flexure and shear. In addition to the neat epoxy, three
loadings of COOH-MWCNTs were examined: 0.5 wt%, 1.0 wt% and 1.5 wt% of epoxy.
While no significant statistical difference in the flexure response of the on-axis specimens
was observed, significant increases in the flexure strength, modulus and toughness of the
off-axis specimens were observed. The average increase in flexure strength and flexure
modulus with the addition of 1.5 wt% COOH-MWCNTs improved by 28% and 19%,
respectively. Finite element modeling is used to demonstrate fiber domination in on-axis
flexure behavior and matrix domination in off-axis flexure behavior. Furthermore, the 1.5 wt%
COOH-MWCNTs increased the toughness of carbon woven composites tested on shear by
33%. Microstructural investigation using Fourier Transform Infrared Spectroscopy (FTIR)
proves the existence of chemical bonds between the COOH-MWCNTs and the epoxy matrix.
Keywords: carbon nanotubes; functionalization; woven fabric; epoxy; FTIR; toughness
Materials 2014, 7 4641

1. Introduction
The use of fiber-reinforced polymer (FRP) composites has grown very rapidly over the last few
decades, due to their attractive physical, mechanical and thermal properties [1]. Some of their common
applications include civil infrastructure, composite bridge decks, oil and gas pipelines and turbine
blades in windmills. The woven fabric composite, in particular, has attracted the interest of engineers
and researchers, due to its relatively high delamination and impact resistance. However, the failure of
FRPs is complex, especially when multi-layer laminates are examined. The failure behavior depends
on many factors, such as the loading nature and direction with respect to the fibers, the fabrication
quality and the structure of the composite fabric. Typical failure modes of FRPs include fiber fracture,
matrix fracture, fiber-matrix interface debonding and interlaminar delamination [2,3]. One of the recent
promising techniques to overcome the premature failure of composites due to delamination/debonding is
to reinforce the composites with nanoparticles.
Carbon nanotubes (CNTs), nano-clay, graphene nanoparticles (GNPs), and nano-silica are common
non-metallic nanoparticles used to fabricate nanocomposites [4,5]. Discovered by Ijima in 1991 [6],
CNTs have attracted many scientists worldwide, because of their distinguished mechanical properties
compared with conventional structural materials. Therefore, many researchers have focused on using the
CNTs in polymer nanocomposites. Over the last two decades, various techniques have been developed to
ensure a good dispersion of CNTs in polymeric matrices, such as non-covalent [7–9] and covalent [10–12]
functionalization. The covalent functionalization involves impregnating functional groups on the surface of
nanotubes. The functional groups are expected to react with the polymer matrix and increase the
interfacial bond significantly. A review for the use of functionalized CNTs in polymeric matrices can
be found elsewhere [13–15].
Several studies reported good improvement in FRP mechanical properties when CNTs were used in
polymer nanocomposites. The mechanism of CNTs in composite is expected to resist the inter-fiber
fracture and enhance their respective mechanical properties. The improvements of FRP composites include
the tensile and shear property of glass fiber-reinforced polymer (GFRP) composites [16] and Mode I
and Mode II fracture toughness [17,18]. For instance, Qiu et al. [16] examined the tensile and shear
behavior of (GFRP) composites. With 1.0% by weight functionalized multi-walled carbon nanotubes
(MWCNTs), they reported a 14% and 5% increase in on-axis tensile and shear strengths, respectively.
They also reported a 20% and 8% increase in tensile Young’s and short beam shear moduli. In addition,
Garcia et al. [18] examined the fracture toughness of carbon fiber-reinforced polymer (CFRP) with
nanotubes joining the prepregs, and they found a 250% increase in Mode I and a 300% increase in
Mode II fracture toughness. Furthermore, compression shear tests were performed to determine the
interlaminar shear strength (ILSS) of glass woven fabric composites with different dispersion techniques
and showed no variation in ILSS due to the addition of 0.5 wt% MWCNTs [19].
Few studies examined using nanoparticles to improve the flexure behavior of FRP composites, such
as increasing the ultimate strength, the flexural modulus or the energy absorption under flexural loads.
Hossain et al. [20] reported a 49% and 31% increase in the flexural strength and modulus of on-axis
woven E-glass/polyester composites reinforced by 0.1–0.4 wt% carbon nanofibers (CNF). In addition,
1.5 wt% aligned MWCNTs are used to improve on-axis flexure strength and modulus for (CFRP)
composites by 74% and 75%, respectively [21]. To date, the effect of using functionalized MWCNTs
Materials 2014, 7 4642

on the flexure behavior of carbon woven fabric composites has not yet been reported. In this investigation,
a carboxyl functionalized multi-walled carbon nanotube (COOH-MWCNTs) epoxy nanocomposite is
used to improve the on- and off-axis flexural behavior of woven carbon fiber composites.
2. Experimental Methods
2.1. Materials
The carbon fiber and the epoxy were supplied by U.S. Composites, Inc., Florida, FL, USA. The
fabric is FG-CARB5750, a balanced plain bidirectional (0°–90°) weave, PAN-based fiber, a 3 k tow
size and 254 µm-thick. The tensile strength of the raw carbon fibers is 4.48 GPa and the tensile
modulus is 231 GPa. The bi-directional plain weave fabric was selected over the other types of weaves
(i.e., twill or satin), because it provides the largest number of interlaces between the warp and filling
threads. Such a large number of interlaces maintains the geometry of the woven fabric cloth and
minimizes any weaving of the fiber bundles that might occur due to rolling the epoxy during the
fabrication process of the FRP composites.
The epoxy used in fabrication is the commercially available EPOTUF® 37-127 epoxy system. The
epoxy resin is a diluted liquid based on bisphenol-A and contains diglycidyl ether (DGEBPA), while
the hardener is an aliphatic amine. The resin to hardener mixing ratio was 2:1; the pot life was 30–45 min
at 80 °F (26.7 °C); the set time was 5–6 h, and the curing time was 24–28 h at room temperature. This
type of epoxy has relatively low viscosity, which facilitates the impregnation of the carbon fibers
during the fabrication process. Furthermore, the epoxy pot life is relatively long to allow adequate time
for the impregnation and the application of the vacuum environment before the epoxy hardens
partially. The tensile strength and tensile elongation for the epoxy are 65 MPa and 25%, respectively.
Carboxyl functionalized COOH-MWCNTs were supplied by Cheap Tubes, Inc., Vermont, VT,
USA. According to the manufacturer’s data, the outer diameter of the CNTs is 20–30 nm, the inner
diameter is 5–10 nm and the length is 10–30 µm with an aspect ratio ranging from 500 to 1000. The
nanotubes were manufactured using the catalytic chemical vapor deposition technique with the purity
being greater than 95 wt%. The functionalization was performed by the manufacturers using a mixture
of non-organic acids. According to the manufacturer, the functionalized MWCNTs had (COOH)
functional groups of 1.23 wt%.
2.2. COOH-MWCNTs/Epoxy Nanocomposite Fabrication
Two different types of treated epoxies were produced to fabricate the carbon woven composites in
this study. The first type was a neat epoxy prepared by mixing the resin with the hardener and applying
the mixture directly to the carbon fabric during the impregnation process. The second type was prepared
by reinforcing the epoxy with functionalized COOH-MWCNTs. The COOH-MWCNTs were added
first to the resin, and the mixture was sonicated for 1.0 h at 40 °C, as shown in Figure 1a. In order to
ensure the chemical reaction between the functional groups on the surface of the nanotubes and the
resin chains, the dispersed mixture was stirred for 2.0 h at 80 °C. The hardener was added after cooling
the resin, and the COOH-MWCNTs/epoxy nanocomposite was further homogenized by mechanical
mixing for 10 min and then used in the composite fabrication. Figure 2a shows the Scanning Electron
Materials 2014, 7 4643

Microscope (SEM) images for well-dispersed 1.0 wt% COOH-MWCNTs in the epoxy matrix. The
image shows the absence of aggregates/agglomeration of MWCNTs as an indication of the effectiveness
of the dispersion process using sonication and mechanical stirring. A close view is shown in Figure 2b
confirming the diameter and their good dispersion of the MWCNTs.
Figure 1. Fabrication of multi-scale epoxy/carbon woven fabric composite specimens:
(a) sonication of COOH-MWCNTs in the epoxy resin; (b) stirring of the COOH-MWCNTs
in the epoxy resin; and (c) the vacuum environment applied to the composite specimens
during the curing of the epoxy.

(a) (b) (c)
Figure 2. SEM image showing the dispersion of 1.0 wt% COOH-MWCNTs in the epoxy
matrix. (a) The general view with the marked area shown at high resolution in (b); (b) the high
resolution view of COOH-MWCNTs in epoxy showing individual carbon nanotubes.

(a) (b)
2.3. Fabrication of the Multi-Scale Woven Fabric Epoxy Composite
Fabrication of the woven fabric composite plates was performed in accordance with ASTM D5687 [22].
The vacuum-assisted hand lay-up technique was implemented to fabricate the composite plates. Ten
layers of fabric were used to fabricate the specimens. The average dimensions of the composite specimens
were 40 mm-long, 19 mm-wide and 2.7 mm-thick. Peel plies, non-porous and porous release films,
breather plies and an aluminum plate were used to ensure straight and compacted composite sheets and
to facilitate the peeling off of the composite after curing. Adequate epoxy impregnation was achieved
using a roller, and a breather ply was placed to form the uniform air paths once the vacuum was
Materials 2014, 7 4644

applied. The same epoxy/carbon fabric weight ratio was used during the fabrication of all composite
specimens in order to maintain the same epoxy weight fraction. The epoxy weight fractions were
obtained by measuring the change in weight between the original carbon fabric before and after epoxy
impregnation and ranged between 30% and 33%. It is important to note that the addition of
COOH-MWCNTs increased the viscosity of the epoxy during the fabrication; therefore, the limitation on
the upper bound of the COOH-MWCNTs content was set as 1.5 wt% of epoxy. The vacuum bag
contents were maintained at a vacuum level of 2.3 × 10−2 Torr for 24 h. This relatively high level of
vacuum applied for long time period ensures the elimination of all air bubbles. Furthermore, to ensure the
consistent production of the composite with different levels of nanomaterials, ASTM D3171 was used for
determining the fiber weight fraction, and the fiber volume fractions for the different composite materials
were found to be 54.3%, 55.5%, 53% and 52.4% for the neat, 0.5%, 1.0% and 1.5% COOH-MWCNTs
epoxy woven fabric composites, respectively, confirming the acceptable consistency in production [23].
Relatively high void contents of 2%, 2.3%, 1.5% and 3% were observed in 0, 0.5 wt%, 1.0 wt% and
1.5 wt% COOH-MWCNTs, respectively, due to the use of the vacuum-assisted hand layup technique in
the fabrication process. The variation in void content is ±0.8%, and the variation in the fiber fraction is
±1.7%. Such variation in void and fiber fractions within ±2.0% was reported in the literature to have an
insignificant effect on the mechanical properties of composites [24–26]. Therefore, the above variations
would not affect our ability to examine the significance of COOH-MWCNTs on the strength and toughness
of carbon fiber composites.
2.4. Mechanical Testing
Three-point bending tests were conducted to examine the on- and off-axis flexure behavior of the
composite specimens. The flexure test setup and geometry of the specimens are shown in Figure 3a,b.
In the on-axis test, the orientation of the fibers was in the span direction, while the fibers were oriented
at a 45° angle with respect to the span direction in the off-axis test. On the other hand, the short-span
shear test was conducted on the on-axis composite plates via four-point bending. The shear test setup is
shown in Figure 3c. The flexure and shear tests were performed on the composite plates using an
MTS® Bionix servo hydraulic system. The machine has a load cell with a capacity of 25 kN and a
maximum stroke of 130 mm. The sampling rate for all of the experiments was 50 Hz, and the data was
collected by the FlexStar MTS® 793 data acquisition system. In each test, the load and displacement
were measured and recorded.
The flexure stress-strains σ −ε were then obtained using Equations (1) and (2) following
f f
ASTM D790-10 [27].
3PL
σ = (1)
f 2bd2
6Dd
ε = (2)
f L2
where Pis the applied load, L is the span length, b is the width of the specimen, d is the depth of the
specimen and D is the deflection of the mid-span. On the other hand, the average shear
stress-strains τ−γ were obtained according to Equations (3) and (4).
Materials 2014, 7 4645

P
τ= (3)
2bd
D
γ = (4)
a
where a is the shear span. In addition to the neat epoxy, three loadings of COOH-MWCNTs were examined.
They are 0.5, 1.0 and 1.5 wt% of epoxy. Three specimens were tested for each COOH-MWCNTs
loading. The flexure and shear strength, modulus and toughness were then evaluated and compared.
Statistical analyses were performed on the test results and the statistical significance of the difference
between the specimens was evaluated using the Student’s t-test assuming a 95% level of significance.
Figure 3. on- and off-axis arrangements for a flexure test: (a) on-axis flexure test;
(b) off-axis flexure test; (c) on-axis shear test.
1.0 mm/min 1.0 mm/min
19 mm 19 mm
Span direction 2.7 mm Span direction 2.7 mm
4 mm 4 mm
Fiber orientation 32 mm Fiber orientation 32 mm
(0º,90º) 4 mm (45º,-45º) 4 mm
(a) (b)
0.25 mm/min
0.25 mm/min
19 mm
2.7 mm
Span direction
24 mm
Fiber orientation
(0º,90º) 4 mm
32 mm
4 mm

(c)
2.5. Microstructural Investigation
To explain the role of functionalized COOH-MWCNTs on the behavior of carbon fibers, Fourier
Transform Infrared Spectroscopy (FTIR) measurements were conducted on 25.4 mm × 25.4 mm samples
cut from CFRP specimens. Three types of samples were compared: neat CFRP samples, CFRP samples
incorporating 0.5 wt% COOH-MWCNTs and CFRP samples incorporating 1 wt% COOH-MWCNTs.
Furthermore, COOH-MWCNTs powder was separately measured. All of the samples were analyzed with
biconical reflectance Nicolet Nexus 670 micro-Fourier Transform Infrared Spectroscopy (Micro-FTIR)
(Thermo Fisher Scientific, Waltham, MA, USA). The FTIR has a continuum microscope with a Globar
source, an XT-KBr beam splitter and a MCT-A detector over a 100 × 100 micron area with a 4 cm-1
Materials 2014, 7 4646

resolution. Spectra were background corrected using a reflective gold slide and converted to absorbance
using the Kramers-Kronig equation, as per the standard FTIR analysis method after Stefanie et al. [28].
2.6. Micromechanical Modeling
In order to understand the failure mode of the on- and off-axis flexure tests, a micromechanical
finite element model is developed under ANSYS® APDL modeling environment (Pennsylvania, PA,
USA) coupled with micromechanical Autodesk® composite simulation analysis (California, CA, USA).
The micromechanical model is based on multi-continuum theory (MCT) for composite analysis. In
MCT, continuum-based (phase-averaged) stresses and strains can be predicted for each constituent
(e.g., fibers or matrix) in finite element (FE) analysis. In addition, the failure criterion for each
constituent of the composite is predicted and displayed separately, allowing a progressive failure to take
place. Previous research proved the failure analysis by MCT to be fairly close to experimental
observations, since it takes into account matrix damage and the associated reduction in stiffness [29–31].
Two 3D FE models for flexure test specimens were constructed. The two models consist of 10 layers of
woven fabric composite with an equal fill and wrap fiber volume fraction and a thickness of 0.27 mm. In
the first model, the fill fibers were set in the span direction, while in the second model, the fill fibers
were set at a 45° angle with respect to the span direction. 3D SOLSH190 elements are used to model the
laminated composite. Table 1 lists all material properties used in the FE simulations. The material
properties of the constituent materials are obtained experimentally [32], from the manufacturer’s data or
using classical lamination theory (CLT).
Table 1. The material properties of woven fabric composite laminates used in FE simulation.
Designation E , GPa E , GPa G , GPa +S , MPa −S , MPa S , MPa υ
11 22 12 11 11 12 12
Lamina 40 * 40 * 3.9 * 555 * −310 † 70 * 0.20 †
Fibers 230 † 50 † – – – – –
Matrix 2.0 * 0.74 ‡ – – – 0.35 †
* The material properties obtained experimentally from [32]; † the material properties obtained from the
manufacturer’s specifications and other sources; ‡ the material properties computed using classical
lamination theory (CLT); E : elastic modulus in the fill fiber direction; E : elastic modulus in the wrap fiber
11 22
direction; G : in-plane shear modulus; +S : ultimate tensile strength in the fill or wrap fiber directions;
12 11
−S : ultimate compressive strength in the fill or wrap fiber directions; S : in-plane shear strength.
11 12
3. Results and Discussion
Figure 4a,b shows the deformed shape of the on- and off-axis flexure tests, respectively. As expected,
the off-axis specimens exhibited higher deflection and less strength than the on-axis specimens. Close
views for the failed on-axis specimens show that failure occurred due to kink/breakage of the fibers in
the compression side (Figure 4c,e). This is expected, since FRP composites are stronger in tension than
they are in compression. In addition, high resolution microscopic images in Figure 4g,h show the kink
and rupture of fibers in the top and bottom surfaces of the on-axis test specimens, respectively. On the
other hand, failure of the off-axis specimens was due to shear-off of the fibers at a 45° angle in the
compression zone, as shown in (Figure 4d,f). This shear-off is attributed to the weak shear strength of the
epoxy matrix.
Materials 2014, 7 4647

Figure 4. Deformation and failure of on- and off-axis flexure composite specimens.
(a) deformation of on-axis flexure specimens; (b) deformation of off-axis flexure specimens;
(c) failure of on-axis flexure specimens; (d) failure of off-axis flexure specimens;
(e) kink/breakage of fibers at the top surface; (f) shear-off of the fibers at the top surface;
(g) high resolution microscopic image for the kink of fibers at the top surface of the on-axis
test; (h) high resolution microscopic image for the rupture of fibers at the bottom surface of
the on-axis test.

(a) (b)

(c) (d)

(e) (f)

(g) (h)
Materials 2014, 7 4648

The flexure stress-strain curves for the on- and off-axis flexure tests are depicted in Figure 5a,b up
to the 5% and 10% strain levels, respectively. For all COOH-MWCNTs loadings, the on-axis flexure
stress-strain curves were linear up to a maximum value corresponding to the flexure strength of the
composite specimens, where the failure in the compression side occurred (Figure 5a). Once compression
failure occurred, the flexure stress was reduced as the fiber breakage propagated through the thickness.
The linearity in the on-axis stress strain curve before failure proves the fiber domination of the on-axis
flexure behavior. On the contrary, a non-linear stress-strain curve associated with a yield-like plateau
was observed in the off-axis flexural behavior (Figure 5b). The significant non-linearity in the off-axis
stress-strain curves is attributed to the matrix domination on the off-axis flexural behavior. In addition,
the post-peak loss in flexure stress occurred in a more gradual fashion in the off-axis behavior than it
occurred in the on-axis direction.
Tables 2 and 3 show the statistical analyses for the mechanical properties of on- and off-axis composite
specimens with various COOH-MWCNTs loadings. The flexure toughness for the on-axis test specimens
were computed from the area under the stress-strain curves up to the 3.0% flexure strain level; the limit
sufficiently extends beyond the fiber breakage first observed at the end of the proportional limit. On
the other hand, the flexure toughness for the off-axis specimens were computed from the area under
the stress-strain curves up to the 5.0% strain level, The proposed limits meet the recommendations of
ASTM D790 [27]. From both tables, it can be observed that the flexure strength (~150–180 MPa) and
modulus (~6–8 GPa) of the off-axis flexure test specimens were about one-half and one-third of that of
the on-axis flexure test specimens, respectively. This is a typical observation supported by composite
theory, where the behavior in the on-axis direction dominated by the strong fibers is much stronger
than off-axis direction dominated by the matrix. The coefficients of variation for all specimens were
below 10%, and therefore, all cases of COOH-MWCNTs loadings showed valid results.
Figure 5. Flexure stress-strain curves for composite plates. (a) on-axis; (b) off-axis.
500 250
Neat 0.5 wt.% COOH-MWCNTs Neat 0.5 wt.% COOH-MWCNTs
1.0 wt.% COOH-MWCNTs 1.5 wt.% COOH-MWCNTs 1.0 wt.% COOH-MWCNTs 1.5 wt.% COOH-MWCNTs
400 200
a
P a
M P
M
ss, 300 s, 150
e s
r e
re St200 e Str100
u r
x u
Fle100 Flex 50
0 0
0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 0 2 4 6 8 10 12 14
Flexure Strain, % Flexure Strain, %
(a) (b)
Based on the Student’s t-test, no significant statistical difference was observed in the on-axis flexure
strength, modulus and toughness between composite specimens with various COOH-MWCNTs
loadings and the neat material. This observation confirms that the on-axis flexural behavior of the
composite plates is dominated by the strong carbon fibers oriented in the span direction, and the
Materials 2014, 7 4649

change/modification in the epoxy matrix produces minimal to no effect on the on-axis behavior. A
similar observation related to the fiber domination on the behavior of woven fabric composites
subjected to on-axis tension was reported earlier by Naik et al. [33] and recently by Soliman et al. [32].
In this case, the composite coupons exhibit fiber breakage associated with limited damage in the
matrix. On the other hand, significant statistical differences in the flexure strength, modulus and
toughness associated with the off-axis flexural test were observed especially with the addition of large
contents of COOH-MWCNTs. For instance, improvements in flexure strength, modulus and toughness
with the addition of 1.5 wt% COOH-MWCNTs reached 19%, 28% and 20.7%, respectively (Table 3).
Table 2. Statistical analyses for the on-axis flexure test with various COOH-MWCNTs
loadings: flexure modulus of elasticity (E), flexure strength (σ ) and flexure toughness at
u
the 3% strain level (T-3%).
COOH-MWCNTs Criterion E (GPa) σ (MPa) T-3% (MPa.mm/mm)
u
Mean 21.70 320 6.70
0 wt% (neat)
STD 0.7 11.93 0.25
Mean (% increase) 20.40 (−6%) 348 (8%) 6.96 (3.9%)
0.5 wt%
STD 0.61 6.24 0.52
Mean (% increase) 19.33 (−10%) 340 (6%) 6.77 (1.04%)
1.0 wt%
STD 0.85 23.90 0.45
Mean (% increase) 20.40 (−6%) 344 (7%) 6.94 (3.48%)
1.5 wt%
STD 0.82 29.28 0.36
Table 3. Statistical analyses for the off-axis flexure test with various MWCNTs loadings:
flexure modulus of elasticity (E), flexure strength (σ ), and flexure toughness at 5% strain
u
level (T-5%).
COOH-MWCNTs Criterion E (GPa) σ (MPa) T-5% (MPa.mm/mm)
u
Mean 6.73 147.7 5.85
0 wt% (neat)
STD 0.29 0.58 0.03
Mean (% increase) 7.00 (4%) 167.7 (13%) 6.05 (3.53%)
0.5 wt%
STD 0.26 4.04 0.21
Mean (% increase) 7.60 (13%) 179 (21%) 6.74 (15.28%)
1.0 wt%
STD 0.78 6.08 0.4
Mean (% increase) 8.00 (19%) 189 (28%) 7.06 (20.70%)
1.5 wt%
STD 0.46 9.54 0.42
Previous research reported that significant damage in the matrix occurs prior to fiber reorientation
and breakage when carbon woven fabric composites are loaded off-axis [32,33]. In this case, the
damage in the matrix is gradual, causing significant nonlinearity in the load-displacement response.
Similarly, the failure behavior of the off-axis flexure test can be therefore attributed to the damage of
the epoxy matrix; thus, the effect of the multi-wall carbon nanotubes is evident. Similar findings on the
significance of carbon nanotubes on the on- and off-axis tension test of composite coupons with carbon
nanotubes were reported elsewhere by the authors [32]. It can be argued that the improvement in the
off-axis behavior is attributed to a strong bond between the COOH functional groups attached to the